Selective copper-catalysed atom transfer radical addition (ATRA) in water under environmentally benign conditions

Simple and green conditions for copper-catalysed ATRA reactions in water have been developed. Firstly, [Cu(ADPA)(H₂O)(ClO₄)₂] (1b, ADPA = 9-[(2,2'-dipicolylamino)methyl]anthracene) was demonstrated to be capable of selectively catalysing the ATRA of CCl₄ to styrene using L-ascorbic acid (AsH₂) as a reducing agent in organic solvent mixtures under ambient atmosphere. Mechanistic investigation suggested that our ATRA reaction proceeded via a single-electron transfer (SET) mechanism through an inner-sphere complex, which is consistent with the widely accepted mechanism for copper-catalysed ATRA. To perform the reaction in water as a sole solvent, a biocompatible surfactant (2 wt% Tween 20 or Tween 80) was added to improve solubility and increase the local concentration of organic reagents and the copper catalyst. Without the need for a complicated oxygen-free set-up, the ATRA reaction catalysed by this simple aqueous-dispersed system can be performed at a mild temperature (60 °C) and a relatively short reaction time (6 h) using 1 mol% of the catalyst. Furthermore, this facile protocol is also applicable for other alkene substrates demonstrated in this work, resulting in satisfactory to excellent substrate conversion and product yields.